A great many different types of aftertreatment systems have been used in connection with internal combustion engines for decades. In many instances, it is desirable to remove particulates in exhaust from internal combustion engines, and exhaust particulate filters or “traps” are widely used for this purpose. While many exhaust particulate filters are quite effective at trapping soot, eventually the quantity of trapped soot reaches a point at which continued operation of the engine becomes problematic or less efficient, or risks damaging the exhaust particulate filter. “Regeneration” is a term generally used to describe the process of cleansing an exhaust particulate filter of trapped soot. One typical approach involves raising the temperature within the filter to a point sufficient to combust the trapped soot and convert it into less undesirable or more readily treated emissions.
A number of different regeneration techniques are well known and widely used. Among these are the use of catalysts resident within an exhaust particulate filter or carried within the engine fuel. Catalysts can assist in combustion of soot at relatively lower temperatures than what might otherwise be required. Other regeneration techniques rely upon injection of a fuel into the exhaust gases, which subsequently ignites upstream of, or upon entering the exhaust particulate filter to increase the temperature therein. Still other techniques utilize in-cylinder dosing or dosing downstream the engine and upstream the filter, to deliver a fuel which raises filter temperature by way of an exothermic reaction without actually igniting. Electrically powered heaters and the like, unconventional engine timing and/or fueling techniques, and back-pressure generating flow restrictors are also used. Regeneration technologies utilizing catalysts tend to be quite expensive, whereas techniques employing electric heaters or specialized engine operation strategies may siphon off energy from the engine. Delivery of fuel into the exhaust gases directly consumes fuel, whereas generating back pressure can reduce the ease with which exhaust gases exit the engine. It will thus be readily apparent that most, if not all, regeneration strategies carry some sort of cost or efficiency penalty.
In many regeneration strategies it is thus desirable to detect an amount of trapped soot within the filter with relative precision and accuracy. On the one hand, it is typically desirable to avoid operating an engine system with an inordinately packed filter, while on the other hand it is desirable to avoid overuse of energy and/or reactant-consuming regeneration strategies. For these reasons, engineers are continually seeking techniques to more accurately and precisely detect an actual amount of trapped soot so that underuse and overuse of regeneration can be avoided. Even seemingly miniscule improvements in detecting soot load, and thus suitable regeneration conditions, can translate into significant real world gains in efficiency.
One general class of soot detection technologies employs electromagnetic energy transmitted through an exhaust particulate filter, and reduced in strength as a portion of the electromagnetic energy is absorbed by trapped soot. These techniques have been known for a number of years, but have yet to achieve their full theoretical potential. Certain of these strategies seek to detect soot based upon observation of phenomena such as frequency shift or other signal attributes in electromagnetic energy transmitted through trapped soot. Others have sought to link the extent of reduction in signal strength to soot amount. These known techniques tend to be computationally challenging, require the use of relatively expensive and complex hardware, or suffer from other shortcomings. Moreover, strategies which appear to perform acceptably in the lab are often discovered to be poorly suited to actual field conditions.
One example strategy leveraging the response of a particulate filter and matter trapped therein to electromagnetic energy is set forth in U.S. Pat. No. 4,477,771 to Nagy et al. Nagy et al. propose exciting a microwave resonant cavity with microwave energy, and monitoring a response of the cavity to sense the effective dielectric constant of material within the cavity to provide a measure of soot content in the filter. As with certain other earlier designs, Nagy et al. appear to rely upon inducing resonance of the chamber, which can be fairly sensitive to filter canister geometry and other factors, and may have certain of the other disadvantages noted above.